Fibrillated acrylic fibre

ABSTRACT

Fibrillated acrylic fibers which are suitable for oxidation in the form of staple fibers are formed as copolymers of acrylonitrile and an unsaturated carboxylic acid such as itaconic acid or methacrylic acid. A further monomer may also be included, such as methacrylate, methylmethacrylate or vinyl acetate. Fibers having a Canadian Freeness value of 350-500, and a decitex value of 1.2 to 1.7 may be produced.

[0001] This invention relates to fibrillated acrylic fibres, and in particular to fibres which are oxidizable to form oxidised acrylic fibres.

[0002] Polyacrylonitrile fibres have been converted commercially to carbon fibres for a number of years. A known process is described in U.S. Pat. No. 3,533,743, in which the acrylonitrile fibre is oxidised prior to carbonisation and graphitisation. The acrylonitrile fibre is typically placed under tension during the oxidation phase of the process so that essentially carbon fibre is made from continuous filament acrylonitrile fibre. Continuous filament carbon fibre is used as reinforcement in composite materials for the manufacture of high-strength/low-weight composites, used for example in fishing rods, tennis racquets and racing car body parts. Where the carbon fibres are used in short lengths for discontinuous fibre reinforcement, these are obtained by chopping the continuous filament into short lengths.

[0003] Fibrillated acrylic fibres are disclosed, for example, in EP-A-0,265,762.

[0004] The present invention provides fibrillated acrylic fibres which are suitable for oxidation in the form of staple fibres, and which are copolymers of acrylonitrile and an unsaturated carboxylic acid.

[0005] Such fibres are particularly suitable for oxidation to form fibrillated oxidised acrylic fibres. Preferably the fibres are copolymers of acrylonitrile and itaconic acid or methacrylic acid. Preferably the fibre contains 0.5-2% itaconic acid groups. The copolymer fibre may comprise a further monomer such as methylacrylate, methylmethacrylate or vinylacetate, or any combination thereof. Typically the copolymer contains 3-8% of the further monomer. The staple fibres have a length of about 4-8 mm and are fibrillated to a Canadian Freeness value of less than 700 and more typically 350-500.

[0006] The fibre has a decitex value of between 0.6 and 6.0 decitex, preferably about 1.2-1.7 decitex.

[0007] Also according to the invention there is provided a method of manufacture of oxidisable acrylic fibre in which a staple acrylic fibre is made by copolymerisation of acrylonitrile and an unsaturated carboxylic acid, and is fibrillated in the gel state prior to drying.

[0008] The staple fibre is cut to a length in the range of 4-8 mm, preferably 6 mm.

[0009] Further monomers such as methyl acrylate, methyl methacrylate or vinyl acetate may be added to the copolymerisation.

[0010] Preferably the acrylic fibre comprises 0.5-2% itaconic acid, 3-8% methyl acrylate, and 90-96.5 of acrylonitrile.

[0011] The itaconic acid or methacrylic acid may be added in the form of free acid or as a sodium or amine salt.

[0012] The invention will hereinafter be described by way of example and with reference to the accompanying drawings in which:

[0013]FIG. 1 is a schematic drawing of various stages in the production of fibrillated polyacrylonitrile fibres, and

[0014]FIG. 2 is a photograph of the fibrillated staple fibres made according to the present invention in Example B after 240 minutes' refining.

[0015] A reaction mixture containing 93.5% acrylonitrile, 5% methyl acrylate and 1.5% itaconic acid (% by weight) is mixed in a feed mix vessel (11) with sodium thiocyanate, accelerators and initiators and is then polymerised in solution in a reactor (12). The polymerised polyacrylonitrile copolymer is then wet spun at a spinning cell (13) into a dilute solution of sodium thiocyanate having a specific gravity of 1.052, stretched, washed and cut (14) into staple fibre having a length in the range 4-12 mm, preferably 6 mm -8 mm.

[0016] The polyacrylonitrile is spun and drawn to produce fibre having a decitex value in the range 0.6-6.0 decitex, preferably about 1.2-1.7 decitex.

[0017] The staple fibre is then mechanically fibrillated in its gel state (never-dried) as a dispersion in the range 0.5-3% fibres by weight in water. The fibre is processed in a suitable fibrillating apparatus (15) such as a disc refiner e.g. a Sprout-Waldron type 105 available from Sprout-Waldron & Co., Muncy, Pa., USA. The fibrillating apparatus basically comprises a pair of disc-like plates about 30 cm in diameter and spaced apart by a variable distance of between 0.2 and 3.75 mm. One plate is fixed and the other plate rotates at a speed of about 3,000 revolutions per minute.

[0018] The dispersion is continuously recycled through the apparatus for a number of cycles to increase fibrillation. In the examples which follow, the refining time is given as a total refining time.

[0019] The through-put through the disc refiner can be varied from 50 liters per minute up to 900 liters per minute.

[0020] The fibrillated loose staple fibre is then passed into an oven (16) for drying, and is then suitable for oxidation. The fibre is opened out (17) and then passed into an oven for oxidation. Oxidation may take place with the opened fibres spread thinly on trays. The polyacrylonitrile (PAN) fibre may be held at 225° C. for two hours to produce a fibrillated oxidised PAN fibre containing about 8% of added oxygen (by weight).

[0021] Such fibres may be used as reinforcing fillers in other compounds.

[0022] The degree of staple fibre fibrillation varied with the processing time as detailed below in the fibrillation tests.

[0023] Fibrillation Test 1

[0024] The fibrillation obtained over different refining time periods is expressed as water retention and imbibition value measured in accordance with DIN 53814. The results are given in Table 1 and Table 2 below. The degree of fibrillation was determined for the following examples in which 6 mm staple fibre was dispersed in 600 liters of water which was continuously passed through a disc refiner at a throughput of 500 liters per minute, with the two plates set 1.0 mm apart. TABLE 1 Water Imbibition Refining time Water Imbibition % (mins) Example A Example B Example C  0 106 106 110  30 111 119  60 111 117 131  90 129 122 120 116 143 115 150 161 114 180 168 179 120 210 200 120 240 181 223 121 300 200 360 240

[0025] Samples A and B were 1.2 decitex Polyacrylonitrile fibres processed in 1.3% and 2% dispersions respectively, and Sample C was a 4.3 decitex fibre processed in a 2% dispersion. TABLE 2 Residual Water Content Residual Water Content % Refining Time Example A Example B Example C 0 115 115 125 240 mins 508 533

[0026] Fibrillation Test II

[0027] Acrylic fibre (6 g, staple length 6 mm) from Example B and demineralised water (2 l) were placed in the bowl of the standard disintegrator described in TAPPI Standard T-205 om-88, and disintegrated (simulating valley beating) until the fibre was well-dispersed. Suitable disintegrators are vailable from Messmer Instruments Limited, Gravesend, Kent, UK and from Büchel van de Korput BV, Veemendaal, Netherlands. The Canadian Standard Freeness (CSF) of the fibre in the resulting slurry or stock was measured according to TAPPI Standard T227 om.97 and recorded in ml. The stock was divided into two 1 l portions for measurement of CSF and the two results averaged.

[0028] The results of CSF value versus refining time are given in Table 3 below. TABLE 3 Canadian Standard Freeness Refining time (min) CSF (ml)  0  60 692 120 646 240 358

[0029] The results of the water imbibition and Canadian Standard Freeness test show a correlation between refining time and increased fibrillation. An increase in fibre content in the dispersion being processed also produces an increase in fibrillation over a given process time. 

1. A fibrillated oxidisable acrylic staple fibre which is formed from a copolymerisation of acrylonitrile and an unsaturated carboxylic acid.
 2. A fibre as claimed in claim 1 wherein the unsaturated carboxylic acid is one of itaconic acid or methacrylic acid.
 3. A fibre as claimed in claim 2 wherein the fibre contains 0.5-2% of itaconic acid units within the polymer chain.
 4. A fibre as claimed in any one of claims 1 to 3 wherein the fibres have a decitex value in the range 0.6-6.0 decitex.
 5. A fibre as claimed in any one of claims 1 to 4 wherein the fibre has a decitex value in the range 1.2 to 1.7 decitex.
 6. A fibre as claimed in any one of claims 1 to 5 wherein the staple fibres have a cut length of 4-12 mm, preferably 6-8 mm.
 7. A fibre as claimed in any one of claims 1 to 6 wherein the fibrillated fibres have a Canadian Freeness value of less than 700 and preferably in the range 350 to
 450. 8. A fibre as claimed in any one of claims 1 to 7 when used as a precursor for oxidation into oxidised acrylic fibre.
 9. A fibre as claimed in any one of claims 1 to 8 the copolymer further comprising at least one further monomer selected from methyl acrylate, methyl methacrylate and vinyl acetate.
 10. A method of manufacture of oxidised acrylic fibres, in which a staple acrylic fibre is made from a copolymerisation of acrylonitrile and an unsaturated carboxylic acid, and then fibrillated in the gel state prior to drying and oxidation.
 11. A method according to claim 10 wherein the staple acrylic fibre is copolymerised from acrylonitrile and itaconic acid.
 12. A method according to claim 10 or claim 11 wherein at least one further monomer selected from methyl acrylate, methyl methacrylate and vinyl acetate, is incorporated into the fibre during the copolymerisation step.
 13. A method according to any one of claims 10 to 12 wherein the fibre is fibrillated to produce a Canadian Freeness value of less than 700, preferably a value in the range 350-700.
 14. A method according to any one of claims 10 to 13 wherein the staple fibre is cut to a length of 4 to 8 mm, preferably 6 mm.
 15. A method according to any one of claims 10 to 14 wherein the fibre is spun to a decitex value in the range 0.6 to 6.0 decitex.
 16. An oxidised fibrillated acrylic fibre made from a fibre as claimed in any one of claims 1 to 9 , or by a method as claimed in any one of claims 10 to 15 . 